Field-effect transistors are well known and find wide applications, for example as a switching device, in memories or in logic circuitry. FET's with semiconductor channels, mostly consisting of silicon (Si) and, more recently, of gallium arsenide (GaAs), have become an established element in data processing systems. Great progress has been made in designing fast and small dimension devices, down to sub-micron structures, that lead to high package densities and thus to high speed integrated circuits. There is, however, a need for still faster circuits.
Investigations have shown that silicon and gallium arsenide semiconductor devices can operate optimally at a temperature of about 77.degree. K., the liquid nitrogen temperature range. This optimum performance at low temperatures, together with the fact that the resistance of metallic wiring or device connections decreases with decreasing temperature, make low temperature systems attractive.
Recording the development of low temperature devices, there have been proposals for semiconductor FET structures having superconductor source and drain electrodes and where the semiconductor current channel, due to the so-called "proximity effect", becomes superconducting in the vicinity of the superconductor electrodes. An article entitled "Three-Terminal Superconducting Devices", written by W. J. Gallagher (IEEE Trans. on Magnetics, Vol. MAG-21, No. 2, March 1985, pp. 709-716) provides a brief description of such proximity effect devices as well as prior art references. Fabrication and operating margins of these devices would, however, be rather critical.
Furthermore, there have been proposals for FET structures comprising a superconductor channel. They have been described, for example, in the following articles: "Superconducting Field-Effect Transistor" by F. F. Fang et al, (IBM Technical Disclosure Bulletin, Vol. 19, No. 4, September 1976, pp. 1461-1462), and in "Experimental Considerations in the Quest for a Thin-Film Superconducting Field-Effect Transistor" by A. F. Hebard et al (IEEE Trans. on Magnetics, Vol. MAG-23, No. 2, March 1987, pp. 1279-1282).
These articles describe studies on structures with a superconductor channel having a thickness of about 10 nm. An applied electric field causes a slight change in carrier density in a thin surface layer at the gate-superconductor interface. This change in carrier density in turn results in a shift in transition temperature T.sub.c in the thin layer. By applying signals to the gate, the thin layer can be switched between "superconducting" and "normal conducting" states. This results in a change in channel resistance.
Since the field-induced effect does not extend deeply into the channel material, various approaches to enhance the magnitude of the effect have been studied and published by A. T. Fiary and A. F. Hebard in two articles "Field-Effect and Electron Density Modulation of the Superconducting Transition in Composite In/InOx Thin Films" (Physica 135 B, 1985, pp. 124-127, North-Holland, Amsterdam) and "Electric Field Modulation of Low Electron Density Thin-Film Superconductors" (Proc. Internat. Workshop on Novel Mechanism of Superconductivity, Berkeley, June 1987). There is another article on this subject by M. Gurvitch et al, "Field Effect on Superconducting Surface Layers of SrTiO.sub.3 " (Materials Research Society 1986, pp. 47-49).
The drawback of these "surface effect" devices is that the change in channel resistance is still quite small. Even in the "switched" thin surface layer the change is only from metal-conducting to superconducting and, in addition, the bulk section of the channel that is not affected by the applied field acts as a metal-shunt. Therefore, the obtainable output signals are too small to be able to drive next stage FET devices.
Another drawback is that the change in T.sub.c is rather small, i.e., operating temperature (T.sub.op) requirements are stringent since, for proper operations, the T.sub.c of the thin layer has to change from "above T.sub.op " to "below T.sub.op ".
At present, the speed of integrated circuits is essentially determined and limited by the relatively high resistance of the wiring and device connections rather than by the devices themselves. Further progress could therefore be achieved if the wiring could be made of superconductor material. At operating temperatures below T.sub.c of the superconductor material, the line resistance would be reduced to zero and systems with devices linked by resistance-free connections offer increased speed.
This has become feasible since the discovery of a new class of high T.sub.c metal-oxide superconductors (also referred to as ceramic superconductors) that were first described by G. Bednorz and K. A. Mueller in their article "Possible High T.sub.c Superconductivity in the Ba-La-Cu-O System" (Z. Physics, Condensed Matter, Vol. 64, 1986, pp. 189-193). Further developments have resulted in metal-oxide superconductor materials, such as YBaCuO and others, having a T.sub.c well above the temperature of liquid nitrogen. One such composition has been described by C. W. Chu et al in an article "Superconductivity at 93K in a New Mixed-Phase Y-Ba-Cu-O Compound System at Ambient Pressure" (Phys. Rev. Lett. 58, No. 9, March 1987, pp. 908-910).
With this development, integrated circuits cooled with liquid nitrogen, and in which both devices and connections consist of superconductor material, are expected to become reality provided high performance devices, e.g., effective switching elements, can be designed. The obstacles encountered in using hybrid semiconductor-superconductor techniques would be removed.